Abstract. Lower stratospheric wind and temperature measurements made from NASA's
high-altitude ER-2 research aircraft during the CRYSTAL-FACE campaign in July
2002 were analyzed to retrieve information on small scale gravity waves (GWs)
at the aircraft's flight level (typically ~20 km altitude). For a
given flight segment, the S-transform (a Gaussian wavelet transform) was
used to search for and identify small horizontal scale GW events, and to
estimate their apparent horizontal wavelengths. The horizontal propagation
directions of the events were determined using the Stokes parameter method
combined with the cross S-transform analysis. The vertical temperature
gradient was used to determine the vertical wavelengths of the events. GW
momentum fluxes were calculated from the cross S-transform. Other wave
parameters such as intrinsic frequencies were calculated using
the GW dispersion relation. More than 100GW events were identified. They
were generally high frequency waves with vertical wavelength of ~5 km
and horizontal wavelength generally shorter than 20 km. Their intrinsic
propagation directions were predominantly toward the east, whereas their
ground-based propagation directions were primarily toward the west. Among the
events, ~20% of them had very short horizontal wavelength, very high
intrinsic frequency, and relatively small momentum fluxes, and thus they were
likely trapped in the lower stratosphere.

Using the estimated GW parameters and the background winds and stabilities
from the NCAR/NCEP reanalysis data, we were able to trace the sources of the
events using a simple reverse ray-tracing. More than 70% of the events were
traced back to convective sources in the troposphere, and the sources were
generally located upstream of the locations of the events observed at the
aircraft level. Finally, a probability density function of the reversible
cooling rate due to GWs was obtained in this study, which may be useful for
cirrus cloud models.